Existing desoldering technologies typically involve use of one or more of hot air (or infrared) reflow equipment, vacuum bulbs or stations, or copper wicks. Reflow equipment is designed to melt all solder joints for a particular electronic component concurrently, facilitating relatively rapid removal of the component from a printed circuit or equivalent board. Reflow equipment does not remove the solder from the board, however, requiring use of another technique to do so before the removed component is replaced.
Vacuum bulbs or stations, by contrast, use suction to remove heated solder directly from joints on boards and transfer the removed solder to suitable waste reservoirs. The suction provided by this technique is not always effective to remove all of the solder from various joints, however, again requiring use of another technique to do so before the selected component can be replaced. Solder removal using vacuum techniques also does not typically or necessarily occur as the solder becomes molten, resulting in excess heat being applied to the board before removal transpires. This excess heat can in some cases damage either or both of the board and its electronic components, potentially degrading performance of the overall circuit.
Solder wicks provide another mechanism for desoldering joints. As disclosed in U.S. Pat. No. 3,627,191 to Hood, Jr., such wicks are generally comprised of copper strands having an exterior capillary surface. In use, the wick contacts a solder joint heated (through the wick) with a soldering iron or other heat source. The heated solder melts and is drawn up onto the wick by capillary forces.
The Hood, Jr. patent additionally discloses use of a now-conventional flux solution as a coating for the solder wick. As discussed therein, a solution of about twenty-five parts by weight of rosin dissolved in about seventy-five parts by weight of a 4:1 volume mixture of methyl acetate to methyl alcohol forms a particularly effective flux. Other noncorrosive fluxes such as the methyl esters of abietic and pimaric acids are also mentioned in the Hood, Jr. patent as potentially effective for various purposes.
Conventional rosinous fluxes can in some cases impart significant residue when used with desoldering wicks to remove solder from printed circuit boards, for example. Because plastic and other coatings for printed circuit boards will not conform to rosinous areas, this residue must often be cleaned before such coating can occur. Tacky rosinous residues are typically aesthetically unappealing and tend to collect dust and metal particles (which can sometimes lead to short circuits), moreover, resulting in other reasons to avoid imparting the residue onto printed circuit boards. Decreasing the residue also promotes diminished use of chlorofluorocarbons (CFCs), the principal cleaning agent currently in use.
Various "low solids" fluxes are commercially available for soldering (rather than desoldering) electronic components, particularly using automatic wave soldering equipment. These soldering fluxes can include compounds that form noxious fumes when heated, essentially limiting use to equipment having appropriate ventilation systems. These soldering fluxes also aggressively react with and corrode copper surfaces (particularly at high humidities and temperatures), potentially degrading the capillary action of desoldering wicks. As a result, they are not employed to coat desoldering braid, which is used to remove solder manually.
One "low solids" flux available for automatic wave soldering is the Superlo Solids 11W Flux, sold as Item No. NR011 by the London Chemical Company, Inc., 240 Foster Avenue, Bensenville, Ill. 60106. According to a Material Safety Data Sheet provided by that company for the product (effective Mar. 15, 1990), Superlo Solids 11W Flux includes 97.7% by weight of volatile materials (such as isopropyl alcohol/2-propanol). Solids, therefore, comprise no more than 2.3% by weight of the flux. The Superlo Solids 11W Flux is currently believed by the applicant to meet or exceed Section 3.2.2 of U.S. Military Specification MIL-F-14256E (Jun. 1, 1989), furthermore, requiring that at least 51.0% of the non-volatile solids be naturally-occurring rosins. Accordingly, it is anticipated that the Superlo Solids 11W Flux includes approximately 1.16% by weight of naturally-occurring gum rosin and up to approximately 1.14% by weight of subliming solid organic acids.
U.S. Pat. No. 5,004,509 to Bristol (incorporated herein by this reference), by contrast, discloses a soldering flux containing a non-subliming dibasic acid mixture composed principally of glutaric acid in a volatilizing organic solvent. According to the Bristol patent, the acid mixture can volatilize during the soldering process, leaving a soldered product essentially free from corrosion-promoting ionic residue. Because the flux includes substantial quantities of succinic acid, however, like many other automatic soldering fluxes it too is subject to forming noxious fumes when heated.
Additionally, experiments conducted with formulations of the Bristol patent required an alcohol carrier to be heated to more than 120.degree. F. before adding the quantities of acids described therein. Otherwise, the acids tended not to solubilize, but rather to precipitate out as the solution cooled. Such required heating of the alcohol carrier increases the possibility of explosion and fire, making the mixture less suitable for large-scale manufacture absent specialized explosion-proof areas and equipment.